The invention relates to a process for producing a surface-hardened workpiece from sintered iron, the pores on the surface being largely closed after sintering by means of a steam treatment and the workpiece then being subjected to surface-hardening.
German Offenlegungsschrift 3,301,541 discloses a process of the above type. For the purpose of closing the pores as a pretreatment for surface-hardening, a steam treatment is carried out here. To improve the dimensional stability, German Offenlegungsschrift 3,301,541 proposes heating the porous workpiece of sintered iron in a steam-containing atmosphere to produce iron oxide layers on the accessible inner and outer surfaces and, in a second stage, to partially reduce these oxide layers and to harden a narrow zone, starting from the outer surface, in the presence of a carbon-releasing fluid. However, the control of this process and hence also the control of the formation or reduction of the oxide layer are very restricted.
The workpieces produced from a sintered iron, that is to say by powder metallurgy, are in general, porous, unless they are compacted by forging or other forming processes. However, porosity has disadvantages, for example with respect to the corrosion resistance which is impaired, since, for example, liquid materials penetrate into the porous workpiece. This then causes reactions with corrosive agents or the like, the property of the workpiece surface being changed. To prevent penetration of such materials into the surface of a porous workpiece, it is known to coat the latter with synthetic resin, wax, an oily substance or the like.
The porosity of sintered workpieces is additionally disadvantageous if it is intended to obtain surface-hardening. The gases used for hardening can penetrate through the pores into the interior of the workpiece, which hardens throughout. To achieve exclusively surface-hardening, a substantial pore closure, which is stable during the hardening process, is therefore necessary. However, the coatings described above are unstable, so that a corresponding impregnation cannot be used for sintered workpieces which have been subjected to heat treatment after sintering.
Heat treatment is carried out above all for obtaining a greater hardness. Known heat treatment processes are carburising, nitrocarburising, nitriding and oxidising. Surfaces of high wear resistance can be obtained by these processes.